Electromagnetic rotary vibrator and damper for a rotary body

ABSTRACT

An apparatus for electromagnetically and rotationally vibrating a rotary body that is supported by bearings. The apparatus comprises a ring-shaped magnet bipolarly magnetized and mounted coaxially on a shaft of the rotary body, a ring-shaped stator having a coil and adapted to generate a rotating magnetic field by controlling an electric current supplied to the coil, and a frequency variable vibration power source adapted to supply alternating electric power to the coil in the stator. The distance between a location of a magnetic pole in the stator and an axis of the shaft is different than the distance between a location of a magnetic pole in the magnet and the axis of the shaft. The magnet and the stator are disposed so as to be adjacent to each other. Thus a rotational exciting force is applied to the rotary body by an interaction of the rotating magnet field generated by the stator with the magnet. This electromagnetic rotary vibrator can be applied to a damper for offsetting the unbalanced vibration of a rotary body.

BACKGROUND OF THE INVENTION

This invention relates to techniques for electromagnetically androtationally vibrating a rotary body supported by bearings, and moreparticularly relates to an electromagnetic rotary vibrator, for a rotarybody, which is adapted to apply a rotational exciting force to therotary body by using a bipolarly magnetized ring-shaped magnet and aring-shaped stator which are adapted to generate a rotating magneticfield by controlling an electric current that is supplied to a coil.This invention also includes a damper for a rotary body which uses theabove-described vibrator.

A rotary machine has a critical speed at which a vibration amplitudeincreases suddenly when a revolution velocity of the rotary bodyincreases to coincide with a natural frequency. A rotary body used topass a critical speed is subjected to a test for the purpose ofminimizing a vibration amplitude generated by the influence of such ause, thereby evaluating the balance of the vibration of the rotary bodyand carrying out a suitable balancing operation. Rotary machines, suchas first-order passing type to N-th order passing type rotary machines,are especially adapted to pass critical speeds to reach a ratedrevolution velocity and are thus subjected to tests so as to pass thecritical speeds, in the same manner as the above-mentioned rotary body,and a suitable balancing operation is carried out because otherwise, arated revolution velocity cannot be attained. A balancing operation fora rotary body is necessarily carried out in this manner.

According to the conventional balancing techniques, a rotary body isinstalled in an operable rotary machine, and unbalanced vibration ismeasured with the rotary machine in operation. On the basis of theresults of measurement, either a balance weight is attached to therotary body, or conversely, the rotary body is shaved, whereby thevibration of the rotary body is suppressed. In an actual rotation test,the determination of the vibrational amplitude and the calculation andcorrection of balance have to be performed at each and every criticalspeed. Namely, very complicated operations requiring extensive labor andtime have to be carried out.

In recent years, a displacement feedback control method using acontrolling type magnetic bearing has been studied as one method ofsolving these problems. A controlling type magnetic bearing isoriginally adapted to apply a force to a rotary shaft so that the rotaryshaft is constantly maintained in a neutral position in the samebearing. In a damping operation using this controlling type magneticbearing, an exciting force offsetting an unbalanced vibration of arotary body, and synchronous with the rotation of the body, is appliedin a superposed manner to the controlling type magnetic bearing so as tosuppress the vibration and allow the body to pass critical speeds sothat balance evaluation can be carried out. Such a method is disclosedin, for example, Japanese Patent Laid-Open Application No.4-351348/1992.

A controlling type magnetic bearing is basically adapted to attract amagnetic substance of a rotary shaft to electromagnetic actuators andretain the rotary shaft in a neutral position in the bearing. Theelectromagnetic actuators, the attractive force of two poles which acton one position, are used by arranging them in four directions of an XYplane (a plane perpendicular to a direction Z in which the rotary shaftextends). In order to generate a rotating magnetic field for applying aexciting force to a rotary body, rotational synchronous single-phaseelectric power of sine and cosine waves is generated by using a personalcomputer-controlled DSP (digital signal processor) or a tracking filtercircuit, and the generated rotational synchronous single-phase electricpower is superposed to the neutral position, which retains electricpower, of the electromagnetic actuators arranged in the XY plane.

However, since the controlling type magnetic bearings are originallyadapted to apply a force to, and control, a rotary shaft so that therotary shaft is retained constantly in a neutral position in thebearings as mentioned above, an exciting force, offsetting unbalancedvibration and synchronous with the rotation of the rotary body,constitutes a disturbance to the magnetic bearings and causes the actionof the bearings to become unstable. Namely, an exciting force due to theeffect of the attractive rotating magnetic field of the controlling typemagnetic bearings is generated for obtaining a spring effect bydisplacement feedback and a damping effect by speed feedback, and theoriginal function of the controlling type magnetic bearings works so asto offset this disturbance (exciting force). Therefore, a very difficultcontrol operation is required to correct the unstable bearings.

The functioning of the electromagnetic actuators which attract themagnetic substance of the rotary shaft and which are arranged in fourdirections of XY plane, work only solely at an instant, and they do notwork with full power, such that an exciting force is small. This causesthe dimensions of the electromagnetic actuators to increase. Regarding acontrol operation, problems such as a feedback delay occur. Although theelectromagnetic actuators basically work so as to attract the rotarybody, an unnecessary rotational force is also produced, and this causeswhirling (self-excited vibration of the rotary body). Moreover, in viewof the DSP response characteristics, the attainment of a high revolutionvelocity and the passage of a high-order critical speed are difficult toachieve in the foregoing.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus which iscapable of electromagnetically applying a rotational exciting forcehaving an arbitrary frequency to a rotary body in an efficient manner.

Another object of the present invention is to provide a damper for arotary body, adapted to offset the unbalanced vibration of the rotarybody by electromagnetically applying vibration of an amplitude equal tothat of the unbalanced vibration and of a phase opposite to that of theunbalanced vibration, to the rotary body.

According to the present invention, there is provided an apparatus forelectromagnetically and rotationally vibrating a rotary body which issupported by bearings. This apparatus comprises a ring-shaped magnetwhich is bipolarly magnetized and is coaxially mounted on a shaft of therotary body, a ring-shaped stator having a coil and adapted to generatea rotating magnetic field by controlling an electric current supplied tothe coil, and a frequency-variable vibration power source adapted tosupply alternating electric power to the coil in the stator. Thedistance between a location of a magnetic pole in the stator and an axisof the shaft is different than the distance between a location of amagnetic pole in the magnet and the axis of the shaft. Thus a rotationalexciting force is applied to the shaft of the rotary body by interactingthe rotating magnetic field generated by the stator with the magnet.

The ring-shaped magnet may be mounted directly on the rotary shaft, oron a static portion of a bearing mounted on the rotary shaft. The magnetis preferably a permanent magnet, and may be an electromagnet. Forexample, there is a structure in which an axially bipolarly magnetizedpermanent magnet constituting the ring-shaped magnet, and a stator aredisposed so that the magnet and the stator are adjacent to each other.Such a structure may also be employed that has a radially bipolarlymagnetized permanent magnet constituting the ring-shaped magnet, and astator which are disposed so as to be adjacent to each other radially inthe same plane.

According to the present invention, there is also provided a damper,using the above-described electromagnetic rotary vibrator, which isadapted to offset unbalanced vibration of the rotary body supported bybearings. This damper comprises an electromagnetic rotary vibratorprovided with a ring-shaped magnet which is bipolarly magnetized and iscoaxially mounted on a shaft of the rotary body, and a ring-shapedstator having a coil and adapted to generate a rotating magnetic fieldby controlling an electric current supplied to the coil. The distancebetween a location of a magnetic pole in the stator and an axis of theshaft is different than the distance between a location of a magneticpole in the magnet and the axis of the shaft. A rotation exciting forceis applied to the shaft of a rotary body by an interaction of therotating magnetic field generated by the stator with the magnet. A meteris provided with a rotation sensor for detecting a revolution velocityof the rotary body. Also provided is a damping power source adapted togenerate a sine wave signal of a frequency which is synchronous with therotation of the rotary body by an output pulse from the rotation sensorand to supply a driving output, which is obtained by controlling thephase and amplitude of the sine wave signal, to the coil of the stator.Thus vibration of a phase, opposite to that of the unbalanced vibrationof the rotary body and of an amplitude substantially equal to that ofthe unbalanced vibration of the rotary body, is generated by theelectromagnetic rotary vibrator.

In this damper for a rotary body, the meter is preferably provided, inaddition to the rotation sensor for detecting a revolution velocity ofthe rotary body, with a vibration sensor for detecting the vibration ofthe rotary body, and a measuring instrument for determining the phaseand amplitude of the vibration on the basis of outputs from the twosensors. The damping power source is provided with a phase locked loopor phase synchronizing loop adapted to generate a signal of a frequencywhich is synchronous with the rotation of the rotary body by an outputpulse from the rotation sensor, a sine ROM for storing sine wave data, aD/A converter adapted to carry out D/A conversions on the basis of thesine wave data read from the sine ROM, and a phase-voltage controlleradapted to control the sine wave data which is outputted from the sineROM, in such a manner that the phase of the sine wave data becomesopposite that of the unbalanced vibration of the rotary body obtainedfrom the measuring instrument, and to control an output from the D/Aconverter in such a manner that the amplitude of the output from the D/Aconverter substantially corresponds to that of the unbalanced vibrationof the rotary body.

In the electromagnetic vibrator for a rotary body, the distance betweena location of a magnetic pole in the stator and an axis of the shaft isdifferent than the distance between a location of a magnetic pole in themagnet and the axis of the shaft, so that a resultant force based on theinteraction, i.e. repulsion and attraction of the magnetic fieldgenerated by the stator and the magnet, is exerted in a planeperpendicular to the axial direction of the rotary shaft. This resultantforce rotates around the shaft in accordance with the rotating magneticfield generated by the stator. Consequently, the rotational excitingforce is exerted on the rotary body. The rotational exciting force canbe controlled freely on the basis of the value, frequency and phase ofthe alternating current supplied to the stator.

When vibration, having a phase opposite to that of the unbalancedvibration of the rotary body and having an amplitude substantially equalto the unbalanced vibration, is applied electromagnetically to therotary body, the unbalanced vibration is offset. For example, a typicalexample is shown schematically in FIGS. 1 and 2. When an electromagneticvibrational response, shown by the broken line, is applied to anunbalanced vibrational response, shown by the solid line, in a simpleharmonic vibration waveform on a time axis as shown in FIG. 1, asynthesized waveform of the solid line and broken line constitutes anoffset waveform. FIG. 2 is a Nyquist diagram which is representative ofthe response shown in FIG. 1, and indicates that a synthesizedvibrational response in which an unbalanced vibrational response and anelectromagnetic vibrational response, the phase of which is opposite tothat of the unbalanced vibrational response, are combined and thus, theunbalanced vibration and electromagnetic vibration offset each other.Therefore, the passage of an N-order critical speed can be performed bypracticing the above-described operation each time the rotary bodypasses a critical speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing a vibration waveform in thevicinity of a critical speed.

FIG. 2 is an explanatory drawing showing an example of a Nyquist diagramin the vicinity of a critical speed.

FIG. 3 is a schematic explanatory drawing showing an embodiment of theelectromagnetic rotary vibrator according to the present invention.

FIG. 4 is an explanatory drawing showing another embodiment of theelectromagnetic rotary vibrator according to the present invention.

FIG. 5 is an explanatory drawing showing still another embodiment of theelectromagnetic rotary vibrator according to the present invention.

FIG. 6 is a schematic block diagram showing an embodiment of the damperaccording to the present invention.

FIGS. 7A and 7B are explanatory drawings showing examples of vibrationcharacteristics before and after a vibration offsetting operation.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 3 shows a basic construction of an electromagnetic rotary vibratorfor a rotary body according to an embodiment of the present invention.This apparatus is adapted to apply a rotational exciting force to anarbitrary rotary body supported rotatably by a passive bearing (notshown), and comprises a ring-shaped permanent magnet 10, a ring-shapedstator 12 having a coil and a vibration power source 14. The permanentmagnet 10 is axially bipolarly magnetized in the axial direction, and iscoaxially mounted on the rotary body 16 (a shaft 18 of the rotary bodyin this embodiment) coaxially. The stator 12 may have the sameconstruction as a regular induction motor, and is adapted to generate asubstantially bipolar rotating magnetic field with an electric currentcontrollably supplied to the coil of the stator. The permanent magnet 10and the stator 12 have different magnetic radii. The distance between alocation of a magnetic pole in the stator and an axis of the shaft isdifferent than the distance between a location of a magnetic pole in themagnet and the axis of the shaft (distance between a position in which amagnetic pole is formed and the axis of the rotary body shaft 18, themagnetic radii of the permanent magnet and the stator being representedby symbols R1, R2 respectively). The vibration power source 14 is afrequency-variable inverter for supplying an alternating electric powerto the coil of the stator 12.

In the structure of the rotary vibrator shown in FIG. 3, the ring-shapedpermanent magnet 10 is magnetized so that, for example, the upper andlower surfaces thereof have an N-pole and an S-pole respectively. Assumethat an S-pole and an N-pole occur at a certain instant in the right andleft side portions respectively in the drawing of the stator 12 due to acurrent supplied to the coil. Consequently, a left-upwardelectromagnetic force due to the repulsive force of the S-poles occursin the right side portion in the drawing, while a left-downwardelectromagnetic force due to an attractive force between the N- andS-poles occurs in the left side portion in the drawing. If they aresynthesized, the components in the axial direction of the permanentmagnet 10 are in opposite directions and they, therefore, offset eachother. However, the components F1, F2 in the direction (in a horizontalplane in the drawing) perpendicular to the axial direction are in thesame direction and are therefore summed up to turn into a eccentricforce. The direction of a magnetic field occurring in the stator 12changes circularly in accordance with the frequency of the alternatingelectric power supplied from the vibration power source 14, so that thedirection of an electromagnetic eccentric force supplied to thepermanent magnet 10 also changes circularly. This eccentric force can becontrolled on the basis of the frequency and value of the alternatingcurrent supplied to the stator 12. Namely, due to the interaction of therotating magnetic field generated in the stator 12 with the permanentmagnet 10, a desired level of a rotational exciting force can be appliedto the rotary body 16, and an electric input generates a high-qualityexciting force of a rotational vibration vector.

The inverter used as the vibration power source 14 is generally set toproduce a rectangular wave having a variable voltage and frequency bytemporarily turning the commercial electric current into a directcurrent, and carrying out the positive-negative voltage switching inaccordance with the frequency. Such a rectangular wave inverter iscommercially available and can be easily obtained inexpensively. Thewaveform of the vibrating AC power may be a sine waveform in addition toa rectangular waveform. However, a sine wave inverter is expensive andnot generally preferred.

The above-described rotary vibrator for a rotary body according to thepresent invention is an optimum embodiment, but a structure for applyinga rotational exciting force to a rotary body is not limited to theabove-described embodiment. For example, although a permanent magnet isused as the ring-shaped magnet in the above embodiment, it may bereplaced by an electromagnet. In this case, a structure having a windingaround a magnetic core (not shown) may be mounted on the shaft of therotary body by supplying an electric current to the winding via a brushas used in conventional DC motors.

It is also possible to dispose the magnet and the stator in the sameplane. An example of such an arrangement is shown in FIG. 4. Aring-shaped permanent magnet 20 is radially bipolarly magnetized (theouter circumferential surface has an S-pole, and the innercircumferential surface an N-pole) and mounted on a rotary body 21. Therotary body 21 is rotatably supported by a bearing 22. A stator 23 isring-shaped, and is adapted to generate a rotating magnetic field withan electric current, which is supplied to a coil and controlledproperly. The permanent magnet 20 and the stator 23 are disposed in thesame plane so as to be adjacent to each other in the radial direction(i.e., the permanent magnet 20 is positioned on the inner side of thestator 23). If a magnetic field generated by the stator in thisstructure at a certain instant is in the condition, for example, asshown in FIG. 4, an attractive force of different poles occurs in a leftportion of the rotary body 21, while a repulsive force of the same polesoccurs in a right portion of the rotary body. Consequently, a leftwardforce is exerted on the rotary body 21. Since this force rotates withthe rotating magnetic field generated by the stator 23, the rotary body21 receives an exciting force which is rotated about the shaft.

Although the magnet is mounted directly on the rotary body in both ofthe above-described examples as shown in FIGS. 3 and 4, it may bemounted via a bearing. Such an example is shown in FIG. 5. A ballbearing 26 is mounted on a rotary body 25, and a ring-shaped permanentmagnet 27 is mounted on a static portion of the bearing 26. In thisexample, the permanent magnet is axially bipolarly magnetized just asthe permanent magnet shown in FIG. 3. A ring-shaped stator 28 isdisposed so as to be adjacent axially to the permanent magnet 27. Suchan arrangement of the magnet mounted via a bearing has the advantage inthat the magnet does not receive rotational stress, so that anelectromagnet can be easily installed. However, since a load is impartedto the bearing, consideration must be given so as to prevent the seizureof the bearing.

A damper is provided for a rotary body as one of applied examples of theabove-described electromagnetic rotary vibrator for a rotary body. Anexample of schematic construction of the damper is shown in FIG. 6. Arotary body 30 to which the damper is applied is supported in ahorizontal plane (XY directions), by upper and lower bearings 32 and 34,each of which has a spring and a damping member, and in a vertical plane(Z direction), by a thrust bearing 36 also having a spring and a dampingmember. The rotary body 30 further has a driving motor 42 comprising apermanent magnet 38 and a stator 40, and is rotated at a desired speedby supplying an electric current from the driving inverter 44 to thestator 40. The damper for the rotary body according to the presentinvention is provided with an electromagnetic rotary vibrator 46, ameter 48 and a damping power source 50, and the damper is formed so asto offset the unbalanced vibration of the rotary body by generating byusing the electromagnetic rotary vibrator, vibration having a phasewhich is opposite to that of the unbalanced vibration of the rotarybody, and having an amplitude which is substantially equal to that ofthe unbalanced vibration.

The construction of the electromagnetic rotary vibrator 46 may bebasically identical with that of the rotary vibrator shown in FIG. 3,and is provided with a ring-shaped permanent magnet axially andbipolarly magnetized and mounted on a shaft 52 of the rotary body, and aring-shaped stator 62 having a coil and adapted to generate a rotatingmagnetic field with an electric current controllably supplied to thecoil of the stator. The distance between a location of a magnetic polein the stator and an axis of the shaft is different than the distancebetween a location of a magnetic pole in the magnet and the axis of theshaft. The permanent magnet 60 and the stator 62 disposed so as to beadjacent to each other axially. A rotational exciting force is appliedto the rotary body by an interaction of the rotating magnetic fieldgenerated by the stator with the permanent magnet.

The meter 48 is provided with a rotation sensor 70 for detecting arevolution velocity of the rotary body, a vibration sensor 72 fordetecting the vibration of the rotary body, and a measuring instrument74, such as an oscilloscope or a FFT (high-speed Fourier transformer),adapted to determine the phase and amplitude of the vibration on thebasis of outputs from the two sensors 70, 72.

The damping power source 50 generates a sine wave signal having afrequency that is synchronous with the rotation of the rotary body by anoutput pulse from the rotation sensor 70, and supplies a driving output,which is obtained by controlling the phase and amplitude of the signal,to the coil of the stator 62 in the electromagnetic rotary vibrator 46.The damping power source in this embodiment is provided with a phasesynchronizing loop or a phase locked loop (PLL) 80 which is adapted togenerate a signal having a frequency that is synchronous with therotation of the rotary body by an output pulse from the rotation sensor70, a sine ROM 82 for storing sine wave data, a D/A converter 84 adaptedto carry out D/A conversions in accordance with the sine wave data readout from the sine ROM 82, and a phase-voltage controller 86 adapted tocontrol the sine wave data outputted from the sine ROM 82, in such amanner that the phase of the sine wave data becomes opposite to that ofthe unbalanced vibration of the rotary body obtained from the measuringinstrument 74, and to control an output from the D/A converter 74 insuch a manner that the amplitude of the output from the D/A convertersubstantially corresponds to that of the unbalanced vibration of therotary body. The phase-voltage controller 86 controls the D/A converter84 on the basis of a signal that is synchronous with the rotation of therotary body and obtained from the phase synchronizing loop 86 and avibrational response which can be determined by the measuring instrument74. In this damper, the vibration condition is grasped by the measuringinstrument 48, and a damping force corresponding to the unbalancedvibration of the rotary body is outputted from the electromagneticrotary vibrator 46 on the basis of the power from the damping powersource 50, whereby the damping of the rotary body is performed.

This damper is of a closed loop control system, and can also be appliedto an open loop control system. The vibration of the rotary body isgrasped by the measuring instrument. The damping of the rotary body canbe performed by controlling the phase and voltage manually using thephase-voltage controller on the basis of the measured vibration. As longas the entire vibration of the rotary body has been grasped in advance,the damping operation can be programmably carried out by controlling thephase-voltage controller. This can be performed even when a damper whichdoes not have a meter is used.

The results of an application of the above-discussed damper to afourth-order flexural critical speed passing type rotary body isschematically shown in FIG. 7. In the damper used in an experiment, amanual control operation of an open loop control system was performed ina stepped manner. FIG. 7A shows the characteristics of a vibrationspectrum before offsetting the vibration, and FIG. 7B shows thecharacteristics of a vibration spectrum after offsetting the vibration.It is understood that the vibration is damped to a level not higher thanan allowable limit of vibration with respect to the first order flexuralcritical speed, as shown by "a," to the fourth order flexural criticalspeed, as shown by "b," using a very simple damper controllingoperation.

The electromagnetic rotary vibrator for a rotary body according to thepresent invention can not only be applied to such a damper, but it canalso be used as a rotation balancer in which the amount of unbalancedvibration is calculated and the balancing of vibration is then performedby a model method based on conventional techniques. The electromagneticrotary vibrator can also be applied to the vibration evaluation in arotating field. (for example, the evaluation of bearing yield strengthor critical speed variation in a rotating field).

In a conventional method for evaluating the yield strength of a bearing,unbalanced vibration is applied as a test weight to a rotary body andthe amount of weight is increased gradually while conducting a rotationtest each time weight is added. Therefore, the number of rotation testsperformed increases, and many process steps are required. When a rotarybody is rotationally vibrated by using the electromagnetic rotaryvibrator according to the present invention, the vibration amplitude canbe increased at each critical speed, i.e., a vibrational response can beattained arbitrarily. This enables a load imparting parameter to be setby one rotation test under high-quality conditions based on theimparting of a dynamic load under the limit conditions of a bearing, andthe yield strength of the bearing can be efficiently evaluated.

When the electromagnetic rotary vibrator according to the presentinvention is used in a rotating field, the variation of the resonancepoint in accordance with an increase in the revolution velocity of arotary body can be detected, and the results of the detection can beimplemented upon the structural evaluation and designing of the rotarybody.

The waveform of the vibrating alternating electric power supplied fromthe vibrating-damping power source may be a rectangular waveform or asine waveform as mentioned above. The vibration evaluation is carriedout for obtaining a vibrational response. Accordingly, the level of aninput may be low, and the waveform thereof may be a rectangular waveformsince data is picked up in an arbitrary frequency band. However,especially when the alternating electric power is applied to a dampingoperation, it is desirable to implement electric power which has a sinewaveform of a single frequency. Vibrating a rotary body with alternatingelectric power having a rectangular waveform, or any other waveformother than a sine waveform, becomes a complex vibrating operation havinga basic frequency and an odd number-multiplied frequency such that, whena critical speed exists in an odd number-multiplied frequency during avibrating operation with a target basis frequency, a vibrationalresponse is made. When an inverter of a rectangular waveform is used,voltage of odd number-multiplied frequencies become smaller in the orderof the multiples, and the levels of exciting force become inverselyproportional to the frequency, such that a vibrational response issmall. However, it should be noted that the region in which the basicfrequency is swept becomes broader. Conversely, if these characteristicsare utilized, a vibrating operation in a high frequency region can becarried out.

The vibrating-damping power source is not limited to the structure shownin FIG. 6. In addition to the above-described method of converting anoutput from a rotary body, there is a method in which a vibrationwaveform is amplified by an amplifier through a synchronizing filter anda phase converter. The methods of vibrating a rotary body on the basisof a vibration-pulse output include (1) a DSP vibrating method using apersonal computer, (2) a vibrating method using an oscillator operatedsynchronously with a trigger, and (3) a vibrating method using aninverter controlled by a personal computer. A rotating magnetic fieldpower can be obtained by various arbitrary methods such as thosedescribed above. In these methods, it is necessary that the function ofregulating the phase and the amount of electric power of a rotatingmagnetic field is provided.

The above description is for a case in which the rotary body makes avibrational response independently at each critical speed as shown inFIGS. 1 and 2. However, depending upon the particular structure of arotary body, there may arise a case in which a plurality of criticalspeeds are approached, so that the rotary body vibrates in a coupledmanner at the same revolution velocity. When a rotary body makes such acoupled vibrational response, it is difficult to carry out an excellentdamping operation by using only a single electromagnetic rotaryvibrator. In such a case, by providing electromagnetic rotary vibratorsin a plurality of arbitrary places and by carrying out controloperations separately, an excellent damping operation can beaccomplished.

As described above, the present invention is for a system in which arotary body is directly vibrated, and the system is capable of applyinga high-quality rotational exciting force of an arbitrary frequency tothe rotary body electromagnetically and efficiently. Therefore, thepresent invention can be used as a rotation balancer for calculating theamount of unbalanced vibration and balancing the vibration by a modelmethod based on conventional techniques. The present invention is alsocapable of setting a load imparting parameter by one rotation test, andbeing utilized as an efficient method of evaluating the yield strengthof a bearing. It is further possible to grasp the variation of aresonance point occurring in accordance with an increase in therevolution velocity of a rotary body, and use the results when designingand evaluating the construction of a rotary body.

Since the present invention is formed so that vibration has an amplitudeequal to that of the unbalanced vibration of a rotary body and has aphase which is opposite to that of the vibration is appliedelectromagnetically to the rotary body, it is possible to damp thevibration by offsetting the unbalanced vibration of the rotary bodywithout substantially requiring a control operation. The performance ofthis apparatus up to the stage of the passage of the existing fourthorder flexural critical speed has already been proven to be excellent.This apparatus can also be applied to an N-order critical speed passingtype rotary body having increased critical speeds, and is very effectivefor an elastic rotary body which is rotated at a high speed. This damperapparatus can also sufficiently perform with the existence oftransitional vibrations and unreproducible vibrations, such as thermalunbalance due to a temperature rise during the rotation of a rotarybody, deformation unbalance due to rotational stress and fluidunbalance.

What is claimed is:
 1. A damper to offset an unbalanced vibration of arotary body that is supported by bearings, comprising:an electromagneticrotary vibrator comprising:a ring-shaped magnet which is bipolarlymagnetized and which is to be mounted coaxially on a shaft of the rotarybody; a ring-shaped stator having a coil, wherein said stator is mountedadjacent to said magnet, and wherein the distance between a location ofa magnetic pole in said stator and an axis of the shaft is differentthan the distance between a location of a magnetic pole in said magnetand the axis of the shaft; and wherein said coil is controllablysupplied with AC power, thereby generating a rotating magnetic fieldthat interacts with a magnetic field produced by said magnet to producea rotational exciting force which is applied to the rotary body; a meterhaving a rotation sensor for detecting a revolution, velocity of therotary body; and a damping power source which receives an output pulsefrom said rotation sensor, generates a sine wave signal which has afrequency that is synchronous with a rotational frequency of the rotarybody, and supplies a driving output to said coil of said stator, whereinsaid driving output is obtained by controlling a phase and amplitude ofsaid sine wave signal; wherein said electromagnetic rotary vibratorgenerates a vibration in the rotary body which has an amplitude that issubstantially equal to an amplitude of the unbalanced vibration andwhich has a phase that is opposite to a phase of the unbalancedvibration.
 2. A damper according to claim 1, wherein:said meter furthercomprises a vibration sensor for detecting the unbalanced vibration ofthe rotary body, and a measuring instrument for determining the phaseand the amplitude of the unbalanced vibration on the basis of outputsreceived from said rotation sensor and said vibration sensor; and saiddamping power source further comprises:a phase locked loop whichreceives the output pulse from said rotation sensor, and generates asignal having the frequency which is synchronous with the rotationfrequency of the rotary body; a ROM for storing said sine wave signalwhich is received from a phase-voltage controller; a D/A converter forperforming D/A conversion of said sine wave signal stored in said ROM;wherein said phase-voltage controller receives outputs from said phaselocked loop and said meter, and accordingly controls said sine wavesignal stored in said ROM so that a phase of said sine wave signal isopposite to the phase of the unbalanced vibration of the rotary bodyobtained from said measuring instrument, and wherein said phase-voltagecontroller controls an output from said D/A converter so that anamplitude of an output from said D/A converter is substantially equal tothe amplitude of the unbalanced vibration of the rotary body.
 3. Anelectromagnetic rotary vibrator for vibrating a rotary body that issupported by bearings, comprising:a ring-shaped magnet which isbipolarly magnetized and which is to be mounted coaxially on a shaft ofthe rotary body; a ring-shaped stator having a coil, wherein said statoris mounted adjacent to said magnet, and wherein the distance between alocation of a magnetic pole in said stator and an axis of the shaft isdifferent than the distance between a location of a magnetic pole insaid magnet and the axis of the shaft; and a frequency-variable powersource which controllably supplies said coil with AC power, therebygenerating a rotating magnetic field, whereby said rotating magneticfield interacts with a magnetic field produced by said magnet to producea rotational exciting force which is applied to the rotary body; whereina bearing having a static portion is mounted on the shaft of the rotarybody, and wherein said ring-shaped magnet is mounted on said staticportion of said bearing.